Fix variant with recent C++ library implementations and cppyy

[saraedum]

Cling (i.e., cppyy) cannot handle the definition of variant as it is provided in recent GCC C++ standard library implementations. We should investigate whether this can be fixed upstream in cppyy-cling somehow. Otherwise, we should probably use a non-standard variant implementation for Side.

See https://sft.its.cern.ch/jira/browse/ROOT-10220 for the upstream issue.

[wlav]

That LLVM upgrade that Axel mentions in ROOT-10220 has not happened yet, though. The hold-up is (AFAIK) a bug that only happens on the CI build machines, but can not be reproduced interactively. Such bugs tend to be spurious memory overwrites, which would be rather bad ... Nevertheless, I'm biting the bullet and moving ahead with the Cling upgrade in cppyy-cling. I'll just split cppyy-2.0.0 and cppyy-1.9.x. Tired of waiting. :)

[saraedum]

This seems to be fixed.

[saraedum]

While the issue in cppyy has been fixed, we still have a workaround in the code that make the left() and right() of a component inaccessible.